adds random runner

- with `lein koan random` a random koan selected to be solved (needs a
  small modification of the lein plugin)
- on success next random koan is selected
- on failure the same koan presented again
- there is an option to skip-koan if the padawan want to proceed to the
  next koan
- if multiple placeholders are present in the koan, multiple solutions
  can be typed in with ';' as delimiter
- the context of the koan is presented and considered for
  placeholders (typically when the koan file contains not fully
  implemented functions, macros)